Mojo Balls are orbs of Supernatural Energy that, when touched, cause the victim to fall in Supernatural rage and attack whatever is nearest. They belong to Grim and were only featured in the Video Game Adaptation of the series. In game, they serve as power ups.
They are first seen by Billy, Mandy, and Grim when Eris and Harold fight over one. Once Grim recognizes the dark orb, he reveals that they are his and that a powerful being had by passed the curse of his Trunk ans released them. The trio then set out on a quest to collect all of the Mojo balls and find the thief. After many battles and a showdown with the Brain Eating Meteor, Mandy and Grim notice that Billy has been leaving trails of chicken feathers so he could find his way home. The former two get suspicious of Billy, in which he shortly after reveals that he had grown a tail of feathers. Grim immediately recognizes this as the curse he had placed on his Trunk and demands to know what Billy's done. He then proceeds to tell his friends that he had only wanted to free his friends "Mo" ans "Jo". The entire cast of the game then quickly gather to beat up Billy for having started this entire problem.
Mojo Balls are globe sized spheres with swirling black and purple energy within.